This invention relates to a cryogenic process for recovering hydrogen and other components from a refinery-type gas mixture. The collection and separation of gases from various refinery operations, such as recovered from crude fractionation, thermal cracking, reforming, catalytic cracking, hydro-cracking, etc., are usually an important part of the overall operation of a petroleum refinery. Gas streams recovered from refinery operations usually contain numerous components of widely differing boiling points, including for example compounds such as hydrogen, methane, ethane, propane, butane, pentane, hexane, cyclohexane, benzene, toluene, and other saturated and aromatic hydrocarbon constituents. Many of these constituents are usually too valuable to be simply burned as plant fuel. For example, hydrogen finds widespread use in the refinery as a reactant, and its recovery can usually be justified.
It is well-known that successful hydrogen recovery by cryogenic techniques requires extremely low temperatures; temperatures on the order of 120.degree. K. (-244.degree. ) are typical. At these extremely low temperatures, most of the heavier components of the typical refinery gas mixture (i.e., those components with high boiling points) will solidify. Consequently, to avoid plugging in heat exchangers and process piping, the concentration of these heavier components must be reduced to very low levels prior to processing the refinery gas mixture at the low temperatures of the cryogenic separation.
In the past, a number of different processing techniques have been proposed for recovering hydrogen from refinery-type gas mixtures using cryogenic techniques. One particularly rudimentary procedure involved simply subjecting the gas mixture to a plurality of gas-liquid equilibrium separations at sequentially lower temperatures. In this way, the heavier, freezing-prone components of the gas mixture could be separated therefrom at the warmer temperatures of the process, and would thereby be prevented from reaching the colder regions of the separation system. Unfortunately, this approach is not only costly but is also sensitive to feed gas conditions, with concentration changes in the feed gas mixture (common with refinery-type gas streams), oftentimes precipitating process upsets.
The most recent approach prior to the present invention, involved an adsorptive pre-purification of the feed gas. In this system, the refinery gas mixture is passed through a compound absorption bed, containing activated carbon and molecular sieve. The gas is first passed through the carbon portion of the bed where the hydrocarbon components posing a problem for the subsequent cryogenic treatment are removed, and the gas is thereafter passed through the molecular sieve portion where primarily water is removed. Although this approach satisfactorily produces a dry, heavy hydrocarbon-free gas mixture suitable for cryogenic treatment, the energy demands of regeneration and the hydrocarbon components lost in the regeneration gas represent undesirable system costs.
In another prior art approach, the gases recovered from the various refinery operations are combined and fed to an absorber column wherein the heavier gas constituents are removed by contacting the gas with a lean oil; generally, a light oil (toluene) is used. The gas stream is thereafter dried and may then be subjected to conventional cryogenic hydrogen recovery techniques. The rich oil recovered from the absorber column may then be treated in an associated stripping column, which may use steam as the stripping gas, to regenerate the lean oil. This operation is not always suitable for preparing a refinery gas stream for cryogenic processing and relatively expensive from the standpoint of both equipment and operating costs.
In a further prior art approach methane and ethylene are condensed from a hydrogen feed stream in a reflux condenser and are used to wash out solid contaminants such as acetylene and carbon dioxide. The gas is introduced into the condenser at a temperature of about 165.degree. K., and the uncondensed vapor is recovered as overhead at a temperature of about 150.degree. K. Operation of the reflux condenser relies upon internally generated reflux.
It is an object of the present invention to provide an improved method for removing potentially freezable constituents from a refinery-type gas mixture which permits the direct recovery, as a separate stream, a major portion of its C.sub.5.sup.+ hydrocarbon constituents.
It is a further object of this invention to provide a method for treating a refinery-type gas mixture that is generally lower in cost than alternate point art systems for accomplishing the same result.
These and other objects readily apparent to those skilled in this technology will become evident from the ensuing disclosure taken in conjunction with the drawing.